Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen

Michal Hocko
2020-01-20T21:27:35Z

On Mon 20-01-20 13:10:56, David Rientjes wrote:
> On Mon, 20 Jan 2020, Michal Hocko wrote:
>
> > > When migrating memcg charges of thp memory, there are two possibilities:
> > >
> > > (1) The underlying compound page is mapped by a pmd and thus does is not
> > > on a deferred split queue (it's mapped), or
> > >
> > > (2) The compound page is not mapped by a pmd and is awaiting split on a
> > > deferred split queue.
> > >
> > > The current charge migration implementation does *not* migrate charges for
> > > thp memory on the deferred split queue, it only migrates charges for pages
> > > that are mapped by a pmd.
> > >
> > > Thus, to migrate charges, the underlying compound page cannot be on a
> > > deferred split queue; no list manipulation needs to be done in
> > > mem_cgroup_move_account().
> > >
> > > With the current code, the underlying compound page is moved to the
> > > deferred split queue of the memcg its memory is not charged to, so
> > > susbequent reclaim will consider these pages for the wrong memcg. Remove
> > > the deferred split queue handling in mem_cgroup_move_account() entirely.
> >
> > I believe this still doesn't describe the underlying problem to the full
> > extent. What happens with the page on the deferred list when it
> > shouldn't be there in fact? Unless I am missing something deferred_split_scan
> > will simply split that huge page. Which is a bit unfortunate but nothing
> > really critical. This should be mentioned in the changelog.
> >
>
> Are you referring to a compound page on the deferred split queue before a
> task is moved? I'm not sure this is within the scope of Wei's patch..
> this is simply preventing a page from being moved to the deferred split
> queue of a memcg that it is not charged to. Is there a concern about why
> this code can be removed or a suggestion on something else it should be
> doing instead?

No, I do not have any concern about the patch itslef. It is that the
changelog doesn't decribe the user visible effect. All I am saying is
that the current code splits THPs of moved pages under memory pressure
even if that is not needed. And that is a clear bug.
--
Michal Hocko
SUSE Labs